1. Field of the Invention
The present invention relates generally to the display of stereo images and more specifically to a system and method for characterizing and adjusting an offset between stereo image pairs for a display environment during playback.
2. Description of the Related Art
Historically, stereo images were displayed using two projectors, one projecting images viewed only by the left eye, and the other projecting images viewed only by the right eye. The left and right eye images were then manually aligned to control the convergence point of the stereo image produced by each left-eye/right-eye image pair projected onto a screen. More recently, the use of single lens stereo projection systems, has made adjusting the convergence point of a stereo image impossible when the stereo image is displayed.
When stereo image content is created, a displayed image size is assumed, and the perceived distance from the viewer where objects appear in a scene (e.g., in front of the screen, coincident with the screen, or behind the screen) is controlled by the author of the stereo image content assuming a predetermined alignment between the left and right eye image pairs. The predetermined alignment corresponds to the assumed displayed image size. One problem with such as approach is that, when the image content is displayed on a smaller or larger screen, the perceived distance of the objects in the scene is affected as a result of an increased or decreased image pair alignment. In some cases, the increased or decreased alignment causes a viewer to experience discomfort due to eye strain or eye fatigue. This problem is further illustrated below in FIGS. 1A-1F.
FIG. 1A is an illustration of a prior art stereoscopic geometry with a convergence depth 111 that equals the display surface depth. A left eye 101 and right eye 102 are separated by an interoccular separation 105. An object that is offset for the left eye image by interoccular separation 105 relative to the right eye image is perceived to appear coincident with display surface 104. In other words, objects that are not separated in the left and right eye images appear in the plane of the display surface. The intersection of the view vectors from left eye 101 and right eye 105 is convergence point 110, which is shown as coincident with display surface 104. During construction of a stereoscopic image, the alignment of left and right eye views for objects in the scene is determined by the author and controls the perceived depth at which the objects appear to a viewer.
FIG. 1B is an illustration of a prior art stereoscopic geometry with a convergence depth 122 that is less than the depth of display surface 104. An object that is offset to the left by stereo image offset 125 in the right eye image compared with the left eye image appears to the viewer at convergence point 120 (i.e., in front of display surface 104). When the right eye image and left eye image are scaled up, the convergence point shifts closer to the viewer. FIG. 1C is an illustration of a prior art stereoscopic geometry for a larger display surface 114 with a convergence depth 123 that is less than the intended convergence depth 122. As shown, as stereo images are scaled to fit the larger display surface 114 (relative to the display surface 104), stereo image offset 125 is scaled to equal larger stereo image offset 124. The viewer may experience discomfort due to increased cross-eyedness as a result of larger stereo image offset 124 since objects in the scene appear at convergence point 121 instead of at convergence point 120.
FIG. 1D is an illustration of a prior art stereoscopic geometry with a convergence depth 127 that is greater than the depth of display surface 104. An object that is offset to the right by stereo image offset 126 in the right eye image compared with the left eye image appears to the viewer at convergence point 123 (i.e., behind display surface 104). When the right eye image and left eye image are scaled up, the convergence point shifts further from the viewer. Objects that are separated by interoccular separation 105 in the left eye and right eye images appear at infinity. FIG. 1E is an illustration of a prior art stereoscopic geometry for larger display surface 114 with a convergence depth 128 that is greater than the intended convergence depth 127 since the separation between the left eye and right eye images is greater than interoccular separation 105. As shown, as stereo images are scaled to fit the larger display surface 114 (relative to the display surface 104) stereo image offset 126 is scaled to equal larger stereo image offset 129 as the stereo images are scaled to fit larger display surface 114. The viewer more than likely may see double images or experience discomfort due to increased divergence as a result of larger stereo image offset 129 since objects in the scene appear beyond infinity instead of at convergence point 123.
Similarly, when the right eye image and left eye image are scaled down, the convergence point shifts closer to the viewer and objects that were intended to appear behind the screen may appear coincident with the screen. FIG. 1F is an illustration of prior art stereoscopic geometry for a smaller display surface 134 with convergence depth 137 that is less than the intended convergence depth 127. As shown, as stereo images are scaled to fit the smaller display surface 134 (relative to the display surface 104) stereo image offset 126 is scaled to equal smaller stereo image offset 136. Again, objects in the scene do not appear at the depths that were intended when the stereoscopic content was created since the stereo image offset used to produce the content does not match smaller stereo image offset 136.
As the foregoing illustrates, what is needed in the art is the ability to modify the alignment between stereoscopic images based on the size of the display surface when a single lens stereo projection system is used.